In order to receive both VHF and UHF TV broadcast signals it has been required to utilize two independent tuners hitherto, where these two tuners have to be switched from one to another. However, recently, an all-channel electronic tuner of double superheterodyne type has been proposed so that all channel signals can be simply selected without switching from one tuner to another. As will be described with reference to the accompanying drawings, such an electronic all-channel tuner has a disadvantage that spurious signals would occur due to harmonics of a local oscillator output signal.
In a tuner of the superheterodyne or double superheterodyne type, mixer or mixers, which convert the frequency of an incoming signal into a desired intermediate frequency, play an important role where the characteristics of mixer(s) have to meet various requirements. Although there are many known compact mixers, in these conventional mixers, image frequency components as well as the harmonic components of the local oscillator frequency are not positively processed or treated. As a result, conventional mixers have to suffer conversion loss of 6 to 10 dB.